This invention relates to decoders for encoded television signals and particularly concerns a modulator for decoding an RF-encoded television signal.
Subscription television broadcasts provide an encoded or encripted television signal that is only viewable in its entirety by subscribers having appropriate decoding apparatus for decoding the encoded signals. One conventional form of encoding amplitude modulates the video with a horizontal line frequency sine wave. This renders the video portion unviewable and also suppresses the horizontal synchronizing signals which contributes to a totally scrambled picture. The audio signal also may be encoded.
Apparatus for unscrambling such encoded television signals generally includes means for producing an appropriately-phased sine wave decoding signal from the received encoded signal and utilizing it to "multiply" the encoded television signal to thereby decode and reproduce the original video signal. One well known decoder circuit applies the recovered sine wave signal to the decoder automatic gain control circuit for varying the gain of the system as an opposite function of the amplitude of the encoding sine wave signal. The result is a decoded output signal. Other prior art circuits merely inject a decoding sine wave signal at the IF input, thus changing the IF signal level as an inverse function thereof, to produce a decoded output signal. Unfortunately, the prior art circuits are non-linear in their effect and result in an impedance mismatch in the IF circuit with a consequent degradation in video frequency response. Further, the decoding effect is generally not linear with changes in RF signal level and the output may vary by as much as 4 dB depending upon the input signal level. A further difficulty is that many of the encripted or encoded television signals are received via cable. Any attempt to decode such signals at RF frequencies must be concerned with impedance mismatches in the subscriber decoder which can result in unusual loading on the cable and undesired signals being backfed thereto.